I know Samuel Huntington is considered outdated and all, but maybe it's interesting to see how his theories apply to the situation.
Briefly: civilizations will clash from time to time. (in this case Sunni and Shiite)
The fighting usually starts in areas with people from mixed civilizations (primary combatants, in this case Sunni and Shiite in Syria and possibly Lebanon).
The countries in the vicinity (secondary combatants) start giving support to their brethren, the conflict expands and the country where the fighting takes place becomes the battleground for these secondary combatants.
The conflict heats up more and the secondary combatants start to ask for support from the most powerful state (or states) within their civilization. The core states.
Enter the Tertiary combatants. The core states usually support their client states in a covert way, but if there is no clear winner they will face a choice: force the client states (secondary combatants) to compromise with their enemies and hammer out a deal, or enter the conflict overtly.
This is often thought too costly, as the core states are now going head-to-head, and if they have nukes, there's no way they'll go there.
So they tell the secondary states to make peace. This forces those states to retract their support for the primary combatants, who feel betrayed, but have no choice but to comply. A deal is struck and the conflict goes back to simmer until it heats up again.
This is from memory, I might not have done Mr Huntington justice with the above summary.
I know, the model is flawed. but I also think he has some of the broad strokes right.
So, applying this. Syria/Lebanon will be the battleground where Iran and Turkey/Saudi Arabia slug it out. It gets bloody and nasty (is already) but when they are about to clash, they will force a peace agreement in the civil war.
As the population of Syria is overwhelmingly Sunni, they will increase their power a lot. No matter how the power sharing agreement is drafted.
Maybe the Shia will get to keep their share of the power, but I don't see how Iran could convincingly win this thing.
A victory for them entails effectively repressing the Sunni majority in Syria, which is undo-able in the long run.
It's just as untenable as Serbia's claim on Kosovo. The demographics make it impossible.
ETA: they seem to skip the middle bit, as one of the core states now seems to have boots on the ground.